Research in optical information recording methods has been advancing in recent years, and such methods have come to be used widely in industrial and consumer applications. In particular, optical information recording media capable of recording information at high densities, such as CDs and DVDs, have become widespread. Such optical information recording media are constructed by layering thin metal films or thermally-recordable thin film materials upon a transparent substrate in which is formed pits expressing an information signal, concavo-convex channels such as guidance grooves for tracking of recording/reproducing light, and so on, and furthermore layering thereupon a protective layer, such as a resin layer or a transparent substrate, that protects the thin metal film or thin film material from atmospheric moisture. The reproducing of information is carried out by irradiating the thin metal film or thin film material with laser light and detecting changes in the amount of the resulting reflected light.
The method for manufacturing such an optical information recording medium is generally performed as follows.
For example, with a CD, a resin substrate having a concavo-convex form expressing information signals on one surface is formed through injection molding or the like using a mold, called a “stamper”, that has a concavo-convex channel pattern on its surface. A thin metal film or thin film material is then formed upon the concavo-convex channels through deposition, sputtering, or the like, after which a protective layer is formed by coating the film with an ultraviolet light-curable resin, thereby completing the manufacture.
With a DVD, a resin substrate approximately 0.6 mm thick is formed through injection molding or the like using a stamper, after which a thin metal film or thin film material is formed upon the concavo-convex form on the resin substrate. This is then laminated onto a separately-prepared resin substrate, approximately 0.6 mm thick, using ultraviolet light-curable resin or the like, thereby completing the manufacture.
Such optical information recording media are seeing increased demand for higher capacities, and due to such demand, higher densities in optical information recording media are being sought. Dual-layer optical information recording medium constructions, in which two signal layers, each formed of concavo-convex channels and a thin metal film or thin film material, are constructed so as to sandwich an intermediate layer tens of μm thick, are offered for the aforementioned DVDs as well, with the goal of increasing the capacity thereof.
Furthermore, next generation optical information recording media, having higher densities and higher capacities than DVDs, are in demand due to the recent spread of digital high-definition broadcasting, and thus high-capacity media such as Blu-ray disks are being offered. Compared to DVDs, the track pitch in the information layers formed in concavo-convex form is narrower in Blu-ray disks, and the pits are smaller as well. It is therefore necessary to concentrate the laser spot used to record/reproducing information into a smaller area on the information layer. With Blu-ray disks, a violet laser whose laser light wavelength is a short 405 nm is used, and the laser light spot is concentrated into a small area on the information layer by using an optical head equipped with an objective lens for laser light concentration whose numerical aperture (NA) is 0.85. However, a smaller spot increases the influence of disk tilt. Aberration will occur in the beam spot with even a slight tilt in the disk, causing distortion in the concentrated beam; this results in a problem in that recording/reproducing cannot be performed. This drawback is circumvented in Blu-ray disks by reducing the protective layer on the laser light-entry side of the disk to a thickness of approximately 0.1 mm.
Meanwhile, in recording/reproducing systems that use optical heads having this sort of objective lens with a high NA, aberration, such as spherical aberration arising due to variations in the thickness from the outer surface of the disk to the information layer, exerts a great influence on the quality of the laser light concentrated onto the information layer. A means for correcting aberration arising due to thickness variations is therefore provided. For example, a configuration that provides a spherical aberration correction means using a combination lens in the optical head, a configuration that provides a spherical aberration correction means using liquid-crystals in the optical head, and so on have been proposed.
Meanwhile, still higher capacities are being demanded even in such high-capacity next-generation optical information recording media such as Blu-ray disks, and thus, as with DVDs, increasing capacity through the multilayering of information layers is being proposed as one such method. In order to reduce the influence of disk tilt when multilayering information layers in a Blu-ray disk, it is necessary for distance to the information layer furthest from the laser light-entry side to be approximately 0.1 mm from the surface of the disk, as with single-layer media. For this reason, the information layers are layered so as to sandwich a transparent layer called an intermediate layer, whose thickness is several μm to several tens of μm, all within a space approximately 0.1 mm thick.
Accordingly, the method of manufacture for a multilayer Blu-ray disk is generally performed as follows. A method of manufacturing a dual-layer optical information recording medium, having two information layers, shall be described as an example. This method includes a step of forming a thin metal film, a thermally-recordable thin film material, or the like upon a molded resin substrate, approximately 1.1 mm thick, having pits, guidance grooves, and so on in a concavo-convex form on one side, thereby forming a first information layer; a step of forming an intermediate layer several μm to several tens of μm thick upon the information layer on the substrate, in order to separate the information layers; a step of transferring pits, guidance grooves, or the like onto the upper side of the intermediate layer by pressing the intermediate layer with a stamper having a concavo-convex form corresponding to the pits, guidance grooves, and so on; a step of forming a thin metal film or thermally-recordable thin film material, the film being semitransparent with respect to the wavelength of the laser light used for recording/reproducing, upon the pits, guidance grooves, or the like transferred onto the intermediate layer, thereby forming a second information layer; and a step of forming a protective layer upon the second information layer in order to protect the second information layer. When multilayering more than two layers, those multiple information layers can be layered sequentially by repeating the steps from the formation of the intermediate layer to the formation of the second information layer several times.
As mentioned earlier, with multilayer Blu-ray disk media constructed in this manner, all information layers are required to be provided within a space approximately 0.1 mm thick in order to reduce the influence of disk tilt. Thus, as shown in FIG. 2, the distance to a first information layer 202, which is furthest from the outermost surface on the recording/reproducing light-entry side, is restricted to approximately 0.1 mm, and the other information layers are layered thereupon moving outward toward the recording/reproducing light-entry side.
While dual-layer media are well-known as such multilayered media, structures having three or more layers are being proposed as of late. In particular, four-layer media, which have four information layers, have been introduced.
When recording/reproducing light is focused onto the information layer to be recorded to/reproduced in an optical information recording medium having multiple information layers, part of the light that has been reflected by another information layer and that is not involved in the recording/reproducing of information (this light is called “stray light” here) is reflected in multiple by one of the information layers. When the stray light returns to the optical head via the same optical path as the reflected light from the information layer being recorded to/reproduced (this reflected light is called “information light” here), the stray light interferes with the information light to be read out, causing major fluctuations in the light amount. Problems caused by such interference are particularly apparent in multilayer media composed of three or more information layers. Light amount fluctuations caused by stray light reflected in multiple returning to the optical head along the same optical path as the information light to be read out shall be referred to here as a “back-focus issue”. Various investigations are being made with respect to the elimination of such back-focus issues.
For example, Patent Document 1 proposes a structure in which the thickness of each intermediate layer is designed so that when light is focused on an information layer to be read, the light does not converge on other information layers. This document particularly discloses a structure in which the thicknesses from one of the information layers to another one of the information layers on the inner side and the thicknesses from that one information layer to one of the information layers on the protective layer side are all different. Making the intermediate layers thicker (or thinner) the further away they are from the recording/reproducing light-entry side is proposed as a way to realize such a structure; this prevents light from converging on other information layers when focusing on the information layer to be read.
In addition, Patent Document 2, for example, discloses a structure for a multilayer medium having three or more information layers in which the intermediate layers are composed having different thicknesses in order to eliminate the influence of crosstalk between information layers (interlayer crosstalk). This document particularly discloses a structure for a four-layer medium having four information layers, in which, when the structure has three intermediate layers, or first, second, and third intermediate layers, that are layered starting with the first intermediate layer, which is furthest from the recording/reproducing light-entry side, and moving out toward the recording/reproducing light-entry side, the second information layer has the highest thickness, thereby preventing stray light from being focused upon other information layers.    [Patent Document 1] JP 2001-155380A    [Patent Document 2] JP 2004-213720A